Meaning
Standardized testing guidelines specify methods for determining the molecular weight and molecular weight distribution of polymers using size-exclusion chromatography. The procedures outlined in the ISO 16014 standard employ high temperature liquid chromatography to separate dissolved polymer chains based on their hydrodynamic volume. This analysis reveals the broadness of the molecular weight distribution, which directly influences the processability and strength of a polymer.
Molecular Separation
The chromatography column is packed with porous gel beads that act as a filtration medium for the dissolved polymer. Shorter polymer chains enter the pores of the beads and take longer to pass through the column, while larger chains cannot enter the pores and elute quickly. Multi angle light scattering detectors measure the concentration and molecular size of the polymer fractions as they leave the column, producing a molecular weight distribution curve.
Viscosity Correlation
Broad molecular weight distributions exhibit strong shear thinning behavior, which allows the polymer to flow easily under high pressure. Narrow distribution resins melt and freeze more abruptly, requiring higher processing temperatures and faster injection speeds to fill thin walled molds. This distribution curve helps compounders design resins with the ideal balance of melt strength and flowability.
Molding Relevance
Variation in molecular weight distribution across resin lots leads to inconsistent parts. If the distribution shifts toward lower molecular weights, the tensile strength and impact resistance of the molded parts drop. This shift is common when recycled regrind is blended with virgin resin, making the test a vital quality gate.
